In an exposure apparatus for semiconductor manufacture, and the like, as a method of detecting the position of the areas of a wafer to be exposed, there is one that discharges light having a predetermined wavelength using an alignment optical system and utilizes the results of the photoelectric conversion of the light reflected from an alignment mark for position detection. Below, a brief description is given of a conventional example of a semiconductor manufacturing exposure apparatus using this technology.
In the exposure apparatus shown in FIG. 5, R designates an original (hereinafter, also called a reticle), on which an electronic circuit pattern is formed, W designates a wafer that is an exposure substrate, and reference numeral 1 designates a projection optical system. In addition, S designates an alignment optical system, 2 designates an alignment illumination device, 3 designates a beam splitter, 4 and 5 designate image formation optical systems, 6 designates an image capturing unit, 7 designates an A/D conversion unit, 8 designates an integrating device, and 9 designates a position detecting unit. Reference numeral 10 designates a wavelength switching unit, 11 designates an XY stage movable in two dimensions, and 13 designates a stage control unit. The stage control unit 13 can position the XY stage at a predetermined position based on detection results of the position detecting unit 9. Reference numeral 14 designates an exposure illumination light source, which illuminates the reticle R. Here, in FIG. 5, although only the alignment optical system S that detects position in the X direction is shown, it is to be understood that, similarly, an alignment optical system for detecting position along the Y axis (a direction perpendicular to the surface of the paper on which the drawing is made) is also provided on the exposure apparatus. The exposure apparatus used for semiconductor manufacture shown in FIG. 5, after relative alignment of the reticle R and the wafer W, emits exposure light from the exposure illumination light source 14 and, through the projection optical system 1, projection-exposes the electronic circuit pattern formed on the reticle R onto the wafer placed on the XY stage 11.
Next, a description is given of a position detecting method of the above-described exposure apparatus. As an initial matter, first, the position of the XY stage is adjusted so that alignment marks WM on the wafer W can be seen in an image capturing screen of the image capturing unit 6. Next, non-exposure light is emitted from the alignment illumination device 2 and the wavelength switched by the wavelength switching unit. A light flux whose wavelength has been switched illuminates the alignment marks WM through the image formation optical system 4 and the beam splitter 3. The light flux reflected from the illuminated alignment marks WM passes through the beam splitter 3 and the image formation optical system 5 and forms an image of the alignment marks obtained from the reflection in an observation area (WP) on the image capturing surface of the image capturing device 6 (FIG. 2E). The image capturing device 6 photoelectrically converts the image of the alignment marks.
Thereafter, an output from the image capturing device 6 is converted into a two-dimensional digital sequence at the A/D conversion device 7. The integrating device 8 converts the two-dimensional signal that is the output of the A/D conversion device 7 into a one-dimensional digital signal sequence S1(x). After conversion to the one-dimensional digital signal sequence S1(x), the position detecting unit 9 detects the positions of the alignment marks WM.
The creation of the one-dimensional digital signal sequence S1(x) by the integrating device 8 necessitates (1) the photoelectric conversion in the image capturing unit 6, and the A/D conversion in the A/D conversion unit 7 must be carried out within the respective dynamic ranges, and (2) that the photodetection levels be sufficient for detection of the positions of the alignment marks WM by the image capturing unit 6. If these two conditions are satisfied, it is possible to carry out detection of the positions of the alignment marks accurately.
A position detection technique that can determined whether or not the image capture detection level is within a predetermined range, as well as that can enable image capturing of a mark at an appropriate photodetection level based on the results of that determination, is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2003-092247.
However, in recent years, the density of semiconductors typified by the DRAM has continued to increase, as has the degree of accuracy required of alignment mark position detection. In order to adjust the photodetection level of the image capturing unit 6, one method adjusts the intensity of the illumination of the alignment illumination device using an ND filter, or the like, not shown, and another method adjusts the image capturing unit 6 photodetection time (that is, the charge storage time of a photoelectric conversion device such as a CCD). However, in a case in which a primary digital sequence S(x) is generated to obtain the S/N ratio of such a sequence S(x) after A/D conversion of the results of photoelectric conversion by the image capturing unit 6, the effects of optical noise caused by flickering of the light source of the alignment illumination device 2 and electrical noise from the image capturing unit 6 and the A/D conversion unit 7 mean that, even if adjustment is made so that the photodetection level is the same, the S/N ratio of the one-dimensional digital signal sequence S(x) varies due to the photodetection level or to the combination of the illumination intensity and the photodetection time. This means that, even with the same photodetection level, the alignment mark position detection accuracy fluctuates, and consequently, in some cases, there is a risk that the alignment mark position detection accuracy deteriorates, and it is possible that there are instances in which simply keeping the photodetection level within a predetermined tolerance is inadequate.